1. Field of the Invention
The present invention relates to an apparatus for inputting coordinates. More particularly, the invention relates to the correction of the output in an apparatus for inputting coordinates which calculates coordinates with the propagation delay time of elastic waves.
2. Related Background Art
There have hitherto been proposed coordinate inputting apparatuses using elastic waves as disclosed in U.S. Pat. No. 4,931,965 and others, for example.
Also, there is disclosed in U.S. Pat. No. 5,070,325 an origin correction method for the coordinate inputting apparatus which has the same structure as above. Further, an apparatus has been proposed to make a correction in the coordinate accuracy in accordance with origin correction data.
In the above-mentioned proposal, the operation of the coordinate calculations is performed as given below.
In a coordinate inputting board on which an X sensor pair S.sub.XL.S.sub.XR and a Y sensor pair S.sub.YU.S.sub.YD, each having two sensors as shown in FIG. 6, are arranged to be orthogonal at the coordinate origin 0, a group delay time tgz and a phase delay time tpz at the origin 0 when vibrations are given to a point P positioned at coordinates (x, y) are provided as origin correction data and stored for each of the sensors. Thus, distances between the input point and each of the sensors and the inputted coordinates (x, y) are calculated as follows: EQU N.sub.i =[{vg(tg.sub.i -tgz.sub.i)-vp(tp.sub.i -tpz.sub.i)}/.lambda.+.alpha.] (1) EQU D.sub.i =N.sub.i..lambda.+vp(tp.sub.i -tpz.sub.i) (2) EQU x=(D.sub.1.sup.2 -D.sub.r.sup.2)/2X.sub.RL +(D.sub.1 -D.sub.r)/2 (3) EQU y=(D.sub.u.sup.2 -D.sub.d.sup.2)/2Y.sub.UD +(D.sub.u -D.sub.d)/2 (4)
where,
tg.sub.i : group delay time for sensors i PA1 tp.sub.i : phase delay time for sensors i PA1 vg: group velocity PA1 vp: phase velocity PA1 .lambda.: wavelength (=vp/f) PA1 f: frequency PA1 D.sub.i : distance between input point and sensors i PA1 X.sub.RL : distance (SXR-SXL) between x sensors PA1 Y.sub.UD : distance (SYU-SYD) between y sensors PA1 .alpha.: time constant integer in [] (0.5 by rounding)
Here, i=1, r, u, and d, which is meant to be the value in each of sensors S.sub.XL, S.sub.XR, S.sub.YU, and S.sub.YD.
In the above-mentioned conventional example, N.sub.i in equation (1) corresponds to the amount of a deviated wavelength of which the detected phase delay time tp is deviated by a certain wavelength, and it is always an integer. N.sub.i is calculated from the difference between the group delay tg and the phase delay tp as expressed in the equation (1) . Therefore, if, for example, the waveform is deformed by an angle at which a vibrating pen is depressed against a vibration transmission member, an offset value is generated due to deviation of the detecting point for the tg, or the like.
FIG. 8 shows such a state as this, indicating the result of actual measurement representing the difference .DELTA.N between N.sub.real arranged into an integer by equation (1) and N already arranged into the integer. Each of lines a, b, c, d, and e corresponds to each of inclinations of pens a, b, c, d, and e shown in FIG. 9. In this result of the measurement, none of them has exceeded the margin for N calculation, i.e., .+-.0.5. Thus, it is apparent that the normal coordinate calculation is possible. However, in the case of the line a in FIG. 8, for example, .DELTA.N fluctuates in center of .+-.0.2. Accordingly, the margin in positive direction is approximately +0.3 which is approximately half +0.5 of the c line. Hence, there is a higher possibility that the accuracy will be lowered due to the fluctuations of the signal level caused by external factors such as noise. Furthermore, in a coordinate inputting apparatus of such kind, the input pen is operated by a person. Usually, therefore the pen is inclined at an angle, not perpendicular, when used. Thus, there is a problem encountered that a sufficient margin cannot be secured.
Also, in the above-mentioned conventional example, the phase velocity Vp can be one of the elements that affect the coordinate accuracy. This velocity Vp is related to the thickness d of a propagation member and a frequency f of vibration and is defined by a function of the product (f.times.d). Therefore, a fluctuation of the thickness upon manufacturing or a variation of the thickness within the propagating member causes the phase velocity Vp to vary, thus resulting in errors in the computation of the distance between a source of vibration and sensor to lower the coordinate accuracy.
Also, as shown in FIG. 29, the delay time tp is not continuous in relation to the distance. For example, the delay time takes the same value t.sub.a even if distances from the sensors are known such as points at distances l.sub.1 and l.sub.2. Thus, it is difficult to calculate the Vp just like distance/time at one point of an optional coordinate position. Therefore, for the measurement of the phase velocity, it is necessary to grasp the relation between the delay time tp and the distance from the vibration source quantitatively as shown in FIG. 29. As a result, the delay time must be sampled at a number of points by varying the distance from the sensor. This leads to increasing the number of manufacturing steps, thus increasing the a cost unavoidably.
Also, in the above-mentioned method of calculating coordinates by detecting both the delay time by a group velocity and the delay time by a phase velocity, there occur distance errors (wavelength errors) of units of wavelength in the distance between the vibrating input source and the sensor when the inclination of the pen or the vibrations of constant velocity cause mainly the group velocity to vary. This will reduce the coordinate accuracy significantly.
In this respect, when the generation of errors is detected and then the coordinating output is stopped for preventing the degradation of the accuracy resulting from such generation of the wavelength errors, the interval of sampling the coordinate becomes too wide, thus creating the problem that its useability for an input operation will be reduced.
Also, in the conventional coordinate inputting apparatus of an ultrasonic type, the detected signal waveform of the wave output from the sensor not only changes in amplitude level due to attenuation of the wave by the distance between the vibrating input pen and the sensor, but also depends greatly on the extent of the tool force, strong or weak, of the input pen exerted by an operator. Therefore, when the delay time is detected at a position as a specific position for signal detection where the detecting signal waveform is beyond a certain constant level (the level is required to be more than a certain constant value for distinguishing from any noise), the leading portion of the detected signal waveform cannot be detected. Instead, depending on the detected signal waveform, its second cycle (FIG. 24B) or third cycle (FIG. 24C) are detected eventually, hence resulting in a drawback that the coordinate is erroneously detected. As a method to solve this, it is possible to specify the level of the detected signal waveform electrically, but the number of circuit parts is increased, leading to a new problem that cost is increased significantly after all. In addition, there is a need for reference signal data to determine its amplification factor. Consequently, not all the inputted data can be used for calculating coordinates. In other words, there is a problem that the sampling rate for the coordinate calculation will be lowered. In consideration of these problems, it is necessary for detection of the delay time to adopt a method which does not depend on the amplitude of the detected signal waveform.
In a coordinate inputting apparatus using ultrasonic waves, it is well known that if the wavelength of the elastic wave which is being propagated on a diaphragm is longer than the thickness of the vibration transmission plate, there is propagated a Lamb wave having different group and phase velocities. When this wave is used, the relations between the distance and the arrival delay time can be schematically shown as in FIG. 25, provided that the detection point for the phase delay time is defined at a point above a given level and also envelope peaks of the detected signal waveform are defined to be the respective detection points for the group delay time. The group delay time presents a relation of greater amplitude of fluctuations, and the phase delay time presents a relation of a stepping state although it is continuous. Such relations are caused by the characteristic of the Lamb wave having different phase and group velocities. In this case, any measurement with accuracy is impossible when the distance is calculated by only group delay time. When the distance is calculated by the phase delay time, the relation between the phase delay time and the distance still remains in the stepping state as shown in FIG. 25 even if a level of the detected signal waveform is made constant electrically to prevent an attenuation of the sonic wave and to remove the dependency of the total force. Therefore, when a value t.sub.0 is output as a delay time in FIG. 25, there arises a problem it becomes impossible to determine whether the distance is l.sub.1 or l.sub.2.